In agricultural machinery for harvesting crops, rough or uneven terrain causes problems in that the height of the cutting mechanism above the ground must be continuously controlled. If the cutting mechanism is set too high, an excessive crop loss may result. If the cutting mechanism is set too low, the cutting mechanism may dig into the ground, possibly causing damage to the cutting mechanism and/or causing dirt to be mixed with the cut crop material as it is fed into the machine. Initially, manual controls were provided to permit an operator to control the height of the cutting mechanism. However, manual controls require constant attention by the operator with a consequent increase in stress.
To alleviate this problem various forms of sensors employing mechanical fingers and optical transmitters/receivers have been used to derive signals for controlling the positioning of a member of an agricultural harvesting machine. The mechanical fingers suffer the disadvantage that weeds may become entangled in them thereby preventing proper functioning. Devices using optical transmitter/receivers have a tendency to fail under dusty conditions. Also, optical devices are unable to sense the ground through weeds and crops.
In a more recent development, ultrasonic transmitters/receivers have been used to develop a positioning control signal. The patents to Thornley et al. (U.S. Pat. No. 4,507,910), Bettencourt et al. (U.S. Pat. No. 4,414,792), Lundahl et al. (U.S. Pat. No. 4,896,486) and Kishida (U.S. Pat. No. 4,663,712) disclose threshing units, combine headers and reapers wherein the cutting unit is position controlled in response to signals developed by an ultrasonic sensor which is fixed relative to the cutting mechanism. In Lundahl et al. and Thornley et al., the ultrasonic signal is directed toward the top of the crop and is reflected therefrom. In Kishida and Bettencourt, the ultrasonic signal is directed toward the ground and the reflected signal is analyzed to determine the height of the sensor, and thus the height of the cutting mechanism above the ground.
While the ultrasonic sensors of Kishida and Bettencourt are capable of measuring the distance between the sensor and the ground under certain conditions, they are not capable of measuring that distance when the ground is covered with weeds. The ultrasonic signal is reflected from the weeds and does not penetrate to the ground thus giving a false indication of the sensor-to-ground distance. It is evident that this is true because the systems of Lundahl et al. and Thornley et al. are both based on the fact that there is a reflection of an ultrasonic signal from crop foliage.
Sound signals below the ultrasonic frequencies might be used to overcome the problem of false reflections from weeds and crop foliage but sound signals best suited for this purpose fall in the range of frequencies characteristic of machine noises generated by the agricultural machine during its operation. Thus, there is a problem of separating the noise from the distance measurement signals.